In a typical implementation, the electrocardiogram (ECG) represents a difference in potential between two or more electrodes placed upon the body of the patient. Leads connect the electrodes to the inputs of a differential amplifier. The differential amplifier detects the ECG signals associated with the contraction of the heart and amplifies the ECG signals so that the ECG signals can be analyzed and/or displayed for analysis.
An external defibrillator, such as an automated external defibrillator (AED), is an example of a device that may analyze and/or display ECG signals. For example, based upon the ECG signals, an AED may assess whether a defibrillation shock is indicated, and charge an energy storage element in preparation for giving the shock. When a shock is indicated, the AED may cue the operator to administer the shock, or the AED may administer the shock automatically. The patient receives the shock through the same electrodes.
It is possible to reduce outside electrical noise associated with ECG signals by applying a third electrode to the patient and connecting the third electrode to a driven reference input of the common mode of the other two electrodes via a driven reference lead circuit. When noise is reduced, the ECG signals are generally easier to analyze. Consequently, as an example, an AED may be more capable of determining whether shock therapy is appropriate and what degree of therapy is appropriate. Further, noise reduction is particularly important in an ECG that is displayed for human analysis. For example, it is generally desirable that a displayed ECG be of “diagnostic quality,” e.g., conform to the standards for diagnostic ECG devices relating to noise, artifacts, and the like, promulgated by the Association for the Advancement of Medical Instrumentation (AAMI).
Another factor that may influence the clarity of the ECG signal is the quality of the connection of the electrodes to the patient. In the case of a patient with a hairy chest, for example, an electrode placed on the chest may lose contact with the patient's skin, resulting in a poor electrical connection. An inadequate electrical connection for one or more electrodes may, for example, result in an inability to detect the ECG signal, or an ECG signal that is not of adequate quality for analysis.